Herceptin cancer drug. Nurse holding a vial containing the powdered form of the breast cancer drug herceptin (trastuzumab) and its packaging. This drug is a monoclonal antibody that binds to HER2 receptors found on the surface of some breast cancer cells. Human epidermal growth factor molecules usually attach to these receptors, making the cells faster growing and more likely to spread. The presence of herceptin prevents growth factor binding and so halts cell growth. It also attracts the body's immune cells, which destroy the cancer cells. About 20% breast of cancers are suitable for treatment with herceptin. In the UK in 2005, it was announced all women with early breast cancer would be tested for HER2 over- expression.